Method of purifying contaminated oil in a centrifugal separator using a separation aid and starting liquid

ABSTRACT

In a centrifugal separator the liquid separation aid attracts/binds contaminating particles in oil supplied to a separation chamber. Purified oil is discharged through a central light phase outlet of the separation chamber, the liquid separation aid and separated particles are discharged through a heavy phase outlet. Purification is carried through using a starting liquid, supplied to the separation chamber to form a layer that creates a liquid seal in the centrifugal rotor, covering said heavy phase outlet. Contaminated oil and liquid separation aid are supplied to the separation chamber. While purified oil leaves the separation chamber through the light phase outlet, at starting liquid and liquid separation aid together with particles are discharged through the heavy phase outlet.

FIELD OF THE INVENTION

The present invention relates to a method of purifying contaminated oilfrom particles suspended in the oil by means of a liquid separation aid,which has a density larger than that of the oil. The separation aid isdispersed in the contaminated oil in order to make the particles moreeasily separable from the oil. The contaminated oil and the liquidseparation aid are supplied into a separation chamber of a rotatingcentrifugal rotor. The liquid separation aid and the particles areseparated from the oil by centrifugal force. The purified oil isdischarged from the separation chamber through a central light phaseoutlet. The separated particles and the liquid separation aid aredischarged from the separation chamber through a heavy phase outletsituated radially outside of the light phase outlet.

BACKGROUND OF THE INVENTION

Mineral oils (also half- and full-synthetic) as well as animal oils andvegetable oils, with or without additives, are used widely in industryfor various purposes, such as lubrication, cooling and insulation.During such use the oils commonly become contaminated by different kindsof particles. Depending on the composition and the particular usedifferent methods for regeneration of contaminated oils are used.

Historically, contaminated oils have been filtered in filter bedscontaining clay, bleaching earth or kieselguhr.

Purification of mineral oils from suspended particles is described forexample in U.S. Pat. No. 4,094,770. According to this patent theparticles that are not filtrated away are removed by addition of anagglomerating aid in the form of a mixture of acetone and 2-butanone.The agglomerated particles settle and may be removed from the oil. Afinishing distillation step is necessary in order to purify the oil fromthe agglomerating aid.

U.S. Pat. No. 4,491,515 describes purification of lubricating oil whichhas been used in vehicles. The oil, which may contain many kinds ofcontaminants, is purified by addition of a compound containing acarbonyl group (ketone) mixed with a water-containing electrolyte, forexample an inorganic or organic acid. When this acid has been added tothe oil, there is relatively rapidly obtained an agglomeration ofparticulate contaminants that may be removed by settling orcentrifugation. The ketone is recovered in a distillation step.

U.S. Pat. No. 4,519,899 describes purification of rolling oil which hasbeen contaminated with particles of the material which has been treatedat the rolling operation. A coagulating agent, as for example awater-containing soda solution, is mixed with the oil in a carefullycontrolled amount. The particles coagulate and are found in the waterphase, which may be removed by settling or in a centrifugal separator.

In SE 512 750 there is described a method for gravimetric separation ofan oil which is contaminated with particles and/or water. According tothis method a collection polymer or polymer mixture, which is notsoluble in oil, is added to the contaminated oil and mixed with thesame, after which separation of the oil and the collection polymeroccurs. The collection polymer and the main part of the contaminantsform a bottom phase, while the oil forms a top phase. The bottom phasewith the collection polymer and the contaminants is removed. Accordingto this publication the separation may take place by centrifugation.

The contaminants in the shape of particles, which are to be removed fromthe oil, may in many cases be very small and difficult to remove fromthe oil. The amount of particles may also be small. The amount ofseparating aid that must be added may therefore be comparatively small.To separate two liquid phases of different density from each other in acentrifugal separator may cause problem if one of the phases is presentonly in a small amount.

SUMMARY OF THE INVENTION

According to the invention, in use of a method as initially defined, theseparation chamber of the centrifugal rotor is pre-charged with astarting liquid, which is heavier than the oil and insoluble therein, inan amount such that a layer of the starting liquid forms a liquid sealin the centrifugal rotor, covering said heavy phase outlet. Only aftersuch pre-charging of the separation chamber the contaminated oil and theliquid separation aid are supplied to the separation chamber, at leastpart of the starting liquid together with liquid separation aid andparticles, separated from the oil, being discharged from the separationchamber through said heavy phase outlet.

The invention may with advantage be carried through in such a mannerthat an amount of the liquid separation aid is used as said startingliquid. In this way only one kind of auxiliary liquid has to be used. Onthe other hand it may be suitable to use a starting liquid of adifferent kind to form the liquid seal in order to restrict the amountof liquid separation aid needed. Such a different kind of startingliquid may be water, for instance.

Among the oils, which may be cleaned by the proposed method, there canbe mentioned oils used for insulation purpose in transformers and tapchangers, rolling oils, hydraulic oils and lubricating oils.

The choice of the liquid separating aid depends on the oil, which is tobe purified. The oils mentioned above may be pure mineral oils, half- orfull-synthetic oils, or animal or vegetable oils with or withoutadditives giving the oils their desired properties: The choice ofseparating aid is also dependent on the kind of contaminating particles,which are to be removed from the oil. As may be understood from theprior art mentioned above different combinations of separating aids areavailable. The liquid separation aid may or may not contain water or besoluble in water. The used separation aid should however be insoluble inoil. Depending on the amount of contaminated particles a larger orsmaller amount of separating aid is added.

The separating aid may contain substances causing flocculation of theparticles, which gives heavier particles more easily removable bycentrifugal separation. The separating aid may also attract or bind theparticles by way of chemical or surface chemical bonds.

By filling a radially outer space in the aforementioned rotor with saidstarting liquid, e.g. liquid separation aid, before oil purification isstarted, an efficient purification can be obtained immediately. If,instead, contaminated oil mixed with liquid separation aid had beensupplied to the rotating rotor from the start, a considerable amount ofoil would be discharged through the separation chamber outlet for heavyphase, before a liquid seal covering this outlet had been formed in therotor by liquid separation aid separated from the contaminated oil.

When the centrifugal rotor contains separation discs improving theseparation, an interface level between oil and separation aid ismaintained, in a steady state process, in the vicinity of the outeredges of the separation discs.

According to the invention a continuous addition of liquid separationaid takes place together with the oil. The liquid separation aid and theoil may, if so is considered suitable, be mixed in some kind of mixerconnected to the inlet of the centrifugal separator or in a separatemixing operation prior to the purification. In the last mentioned case adesired holding time for the mixture of oil and liquid separating aidmay be obtained prior to the purification.

The method according to the invention may advantageously be used forpurification of mineral or synthetic oils containing additives in orderto give the oil the desired properties for the intended application, atwhich the density of the oils lie in the interval of 0.85-1.05 g/cm³ at40° C.

The method of the invention may with advantage be used for mineral oils,20 which have been contaminated with very small particles, for examplevery small soot particles or metal particles which are floating in theoil and therefore do not settle. Mineral oils have usually a density of0.85-0.90 g/cm³ at 40° C. Earlier the only possibility to purify oilcontaining this kind of contaminants have been to use kieselguhr orbleaching clay filters, which filter materials are expensive and createproblems in connection with deposition of the used filter beds.

If the mineral oil has been used as an insulating agent in a transformeror tap changer and is free from additives apart from a necessaryoxidation inhibitor, the separation aid with advantage may be a liquidpolymer, which binds the small soot particles that are present in theoil. Also rolling oil as well as chlorine-paraffin oil contaminated withvery small metal par-tides may be cleaned in this way.

The oil to be purified may also be lubricating oil for Diesel engines,contaminated with small dispersed particles, the separation aid being aliquid polymer.

One example of a suitable liquid polymer for purification of these kindsof oils is a polyhydroxy alkoxylate with a density of 1.0-1.1 g/cm³ at40° C.

EXAMPLE 1

A mineral oil that has been used as an insulating agent in a tap changerand is contaminated with soot particles should be purified from thesame. The amount of oil in the tap changer may be 200-1500 litres. Forthe purification there is used a mobile centrifugal separator MIB303S-13from Alfa Laval AB. Depending on the time interval between consecutivepurification operations the oil contains 1-10% soot. The rotor of thecentrifugal separator is started and is brought to rotate at full speed.Separation aid in the form of a liquid polymer is added to the rotor inan amount of 0.71. The polymer is forced to flow to the outer part ofthe separation chamber of the rotor by centrifugal force, where it formsa liquid layer rotating with the rotor and covets a heavy phase outletof the separation chamber. The amount of polymer that is needed to formthe liquid layer depends on the size and construction of the centrifugalseparator. When the layer has been formed the oil that is to be purifiedis supplied, oil having then already been mixed with polymer. The amountof polymer in the supplied mixture is around 4%. The polymer that isused for the purification consists of a polyhydroxy alkoxylate.

EXAMPLE 2

Rolling oil consisting of a chlorine-paraffin oil shall be purified bythe method according to the invention. The oil, which may contain 1-17%sludge, is mixed with liquid polymer, a polyhydroxy alkoxylate. A mobilecentrifugal separator of the same kind as used in example 1 is used forthe purification. Polymer is added to the centrifugal separator when theseparator has been started to form the rotating liquid layer. The amountof oil that is to be purified may be between 3-15 m³. Previously,contaminated oil had to be deposited at extremely high costs.Purification according to the technology now suggested, therefore, hasimportant advantages.

EXAMPLE 3

Lubricating oil used for lubrication of Diesel engines shall be purifiedaccording to the invention. The oil contains a number of additives inorder to give the oil the desired properties.

The oil is contaminated with 0.1 to 5% of particles mainly consisting ofsoot, combustion rests and reaction products from some of the additivesof the oil. The main part of the contaminants is present as colloidal orsub micron particles, which are impossible to remove in centrifugalseparators or filters.

50 litres of the contaminated lubricating oil is mixed by stirring withabout 4% liquid polymers, a polyhydroxy alkoxylate. The mixture isheated to a temperature of 95° C. In order to separate the polymerentraining the contaminants use is made of a centrifugal rotor of thesame kind as in example 1, which is pre-charged with a polymer layerbefore the oil/polymer mixture is supplied into it.

The result of the extraction was measured by analysis of the oil priorto and after the purification in respect of insoluble matter in n-pentan(ASTM D 893-892).

The amount of contaminants in the oil had decreased by 82%, from 0.96%to 0.18% insoluble matter.

BRIEF DESCRIPTION OF THE DRAWING

The method according to the invention is further described below withreference to the attached drawing, which shows a centrifugal separatorsuitable for carrying through the method of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The centrifugal separator shown in the drawing has a stationary housing1, in which there is arranged a centrifugal rotor 2 rotatable about avertical centre axis R. The rotor 2 is mounted on top of a verticaldriving shaft 3 coupled to a driving device (not shown). A connectiondevice 4 carried by the housing 1 includes, among other things, avertical inlet pipe 5 extending from above into the rotor 2 and formingan inlet channel that opens into a central inlet space in the rotor.Said inlet channel communicates at its upper end with an inlet 5 a ofthe connection device 4.

The rotor 2 forms a separation chamber 6 and several inlet passages 7leading from said central inlet space in the rotor to respective inlets8 of the separation chamber 6. The rotor 2 also forms a central lightphase outlet 9 of the separation chamber 6 at a relatively smalldistance from the centre axis R and, at a greater distance from thecentre axis R, a number of heavy phase outlets 10 of the separationchamber 6.

The light phase outlet 9 is in the form of an overflow outlet leading toan outlet chamber 11 in the upper part of the rotor 6. In this outletchamber 11 there is arranged a paring disc 12, which is supported by theconnection device 4 and has at least one paring channel 13 communicatingwith an outlet 14 of the connection device.

The heavy phase outlets 10 of the separation chamber communicate throughchannels 15 with an overflow outlet 16 of the rotor, opening to a space17 in the housing 1 below the rotor 2.

Within the separation chamber 6 there is a stack of conical separationdiscs 18, which are arranged coaxially with the centre axis R anddelimit between themselves thin passages for through flow of liquidunder treatment in the rotor.

The centrifugal separator in the drawing operates in the followingmanner when used for cleaning oil from small particles suspended thereinby means of a separation aid in liquid form, e.g. a liquid polymer. Theseparation aid is insoluble in the oil to be cleaned and has a densitylarger than that of the oil.

Either before or after the rotor has been brought in rotation about thecentre axis R, a certain amount of separation aid, or another startingliquid heavier than the oil to be cleaned, is supplied through the rotorinlet into the separation chamber 6. This starting liquid forms, whenthe rotor is rotating, a liquid layer in the radially outermost part ofthe separation chamber, covering the heavy phase outlets 10 thereof.

After the separation chamber 6 has been pre-charged with said amount ofa starting liquid, a mixture of oil to be cleaned and said liquidseparation aid is supplied into the separation chamber at anypredetermined rate, preferably continuously. The mixture enters theseparation chamber though its inlets 8 and flows axially through aligneddistribution holes in the separation discs 18 into the variousinterspaces between the discs. In these interspaces, or flow paths, theoil part of the mixture flows inwardly towards the centre axis R,whereas the dispersed separation aid, together with particlescontaminating the oil, moves outwardly away from the centre axis R.

Oil having been freed from particles and separation aid leaves theseparation chamber 6 through its central light phase outlet 9 and flowsfurther on, via the outlet chamber 11 and through the paring disc 12,out through the outlet 14 of the connection device 4. The dispersedseparation aid and the particles move out into the aforementioned layerof starting liquid, which may also be liquid separation aid. From theouter part of the separation chamber 6 at least part of the startingliquid, together with separation aid and particles, separated from theoil, leave the separation chamber through its heavy phase outlets 10 andflow further on, via the outlet channels 15, to and out through theoverflow outlet 16 of the rotor 2 to the space 17 below the rotor.

At the time when said starting liquid has been supplied into theseparation 20 chamber 6, but before any oil to be cleaned has beensupplied, the layer of starting liquid forms an inner cylindrical liquidsurface at a first radial level in the separation chamber. Then, whenoil to be cleaned is supplied, the cylindrical surface of the startingliquid is displaced radially outwardly to a second level. This secondlevel should be situated radially inwardly of the heavy phase outlets10. Thus, the amount of starting liquid to be supplied must beaccurately determined, so that this liquid can form a liquid seal in therotor during the initial part of the separation process, preventingescape of oil through the outlets 10.

In a steady state separation process there is always an interface layerformed between oil and separation aid having been separated from theoil, said interlace layer being maintained at a level radially inside ofthe heavy phase outlets 10.

The centrifugal rotor 2 in the drawing has a solid wall surrounding theseparation chamber 6. However, it may be desirable sometimes to use,in-stead, a rotor having peripheral sludge outlets adapted to beintermittently openable during operation of the rotor, so that particlesseparated from the oil but not entrained by the liquid separation aidleaving through the heavy phase outlets 10 can be intermittentlydischarged during operation of the rotor.

1. A method of purifying contaminated oil from particles suspended therein by means of a liquid separation aid having a density larger than thatof the oil and being dispersed in the contaminated oil in order to makethe particles more easily separable from the oil, the method comprisingsupplying said contaminated oil and said liquid separation aid into aseparation chamber of a rotating centrifugal rotor, separating in saidseparation chamber the particles and the liquid separation aid from theoil by centrifugal force, discharging purified oil from the separationchamber through a central light phase outlet thereof and dischargingseparated particles together with separated liquid separation aid fromthe separation chamber through a heavy phase outlet of the separationchamber, situated radially outside said central light phase outlet,pre-charging the separation chamber, before supplying a substantialamount of contaminated oil thereinto, with a starting liquid, which isheavier than the oil and insoluble therein, in an amount such that alayer of the starting liquid forms a liquid seal in the centrifugalrotor, covering said heavy phase outlet, supplying thereafter saidcontaminated oil and said liquid separation aid into the separationchamber, and discharging from the separation chamber through said heavyphase outlet at least part of said starting liquid and particlestogether with liquid separation aid, separated from the oil.
 2. A methodaccording to claim 1, wherein an amount of said liquid separation aid isused as said starting liquid.
 3. A method according to claim 1, whereinthe oil is a mineral or synthetic oil containing additives giving theoil desired properties for its intended use, the density of the oilbeing in the interval to about 0.85 to about 1.05 g/cm³ at 40° C.
 4. Amethod according to claim 3, wherein characterized in that the oil is apure mineral oil having a density of about 0.85 to about 0.90 g/cm³ at40° C.
 5. A method according to claim 4, wherein the mineral oil is onethat has been used as an insulating agent in a transformer or tapchanger, is free from additives, apart from necessary oxidationinhibitor, and is contaminated with very small soot particles, theseparation aid being a liquid polymer.
 6. A method according to claim 5,wherein that the polymer is a polyhydroxy alkoxylate having a density of1.0-1.1 g/cm³ at 40° C.
 7. A method according to claim 3, whereincharacterized in that the oil is one that has been used as a lubricationoil for Diesel engines and is contaminated with small dispersedparticles, the separation aid being a liquid polymer.